1. Field of the Invention
The present invention relates to systems and methods for improving the performance of sensors used to monitor the exhaust of an internal combustion engine. These gas or fluid sensors include exhaust gas and soot sensors.
2. Background Art
Exhaust gas and other fluid sensors are used for both control and monitoring of internal combustion engines including vehicles powered by gasoline or diesel fuel and using various engine technologies, such as lean-burn, for example. Various types of gas or fluid sensors may include heated exhaust gas oxygen (HEGO) or lambda sensors, universal exhaust gas oxygen (UEGO) sensors, nitric oxide and nitrogen dioxide (NOx) sensors, ammonia (NH3) sensors and soot sensors, for example. These sensors provide information regarding the presence and/or concentrations of particular substances or compounds in the exhaust gas. This information is used by the engine and/or vehicle controller to monitor and/or control the engine.
The standard mounting location of an on-board vehicle exhaust gas sensor is at or near the wall of the exhaust pipe where the exhaust flow is more easily accessible and typically cooler than at the center of the exhaust pipe. As such, the sensor is exposed only to the exhaust gas in this limited region of the pipe, which for many applications is not problematic. However, the present inventors have recognized that the presence or concentration of the component measured by the sensor may not be uniformly distributed across the diameter of the exhaust pipe for some applications or operating conditions. For example, in applications employing a urea/SCR after-treatment system, an ammonia (NH3) sensor may be desirable to detect ammonia (NH3) desorbed or released by the SCR catalyst, the amount of which is very sensitive to the exhaust gas temperature, which is generally higher in the center of the exhaust flow. The present inventors have observed that the concentration of ammonia in the center of a four-inch exhaust pipe may be in certain circumstances from 10 to 100 times greater than it is at the pipe wall where the sensor is traditionally mounted such that the exhaust flow at the pipe perimeter is not necessarily representative of the content of ammonia in the bulk flow. Similarly, for diesel applications using a soot sensor positioned downstream of a particulate filter, soot generated by a crack in the filter, for example, may produce a localized, non-uniform soot distribution difficult to detect using a perimeter mounted sensor.
Laboratory equipment used to analyze exhaust flow often includes a sampling probe with multiple inlets that extends into the exhaust pipe or tube and extracts a sample using a vacuum pump for subsequent analysis. While this approach works well for research and development efforts, the additional complexity, cost, and packaging requirements are not amenable to real-time or near real-time sensing of exhaust flow on-board a vehicle.
Various types of on-board exhaust sensors include a protective tube or shield that surrounds the sensing element to protect or shield it from the harsh environment of the exhaust flow, such as described in U.S. Pat. Nos. 6,637,254 and 6,551,498, for example. Such devices generally allow only a small portion of the exhaust to pass over the sensing element to protect the sensing element while detecting the desired substance in the bulk flow. To protect the sensing element from high exhaust gas temperatures, the sensing element may be positioned some distance away from the exhaust flow as generally described in “Using a MISiCFET device as a cold start sensor” by H. Wingbrant et al., Sensors and Actuators, B93 (2003), pp. 295-303, for example.